The present invention relates in general to detecting the shift in the baseline in an electrocardiogram (ECG) and more particularly concerns a novel system using a multiple element electrode at a single measuring location with means for detecting potential differences between elements to reliably detect the occurrence of a baseline shift, thereby preventing automatic monitoring equipment from indicating an alarm condition about a patient being monitored when an alarm condition should not be indicated. The apparatus is relatively easy to fabricate and install, adds relatively little cost to the system and operates relatively reliably with little attention from maintenance personnel.
Automatic ECG monitoring systems, whether using simple rate alarms on a bedside monitor or more complex arrhythmia detectors, produce erroneous results at times because of the presence of a non-ECG artifact, consisting of a combination of high frequency "muscle noise" and lower frequency "baseline shift". The present invention is concerned with detecting baseline shift; that is, a shift in the average value of the ECG signal over a waveform portion.
The usual cause of "baseline shift" is physical manipulation of the electrodes or the skin and muscle underlying electrodes which may result from movement of an electrode, pressing of the surfaces as a result of patient movement, tugging upon the electrode lead and other causes. Good preparation of the skin surface to which the electrode is attached helps minimize baseline shift but does not eliminate it. As an example of one approach to dealing with this problem reference is made to U.S. Pat. No. 3,905,364 entitled ARTIFACT DETECTOR.
The prior art includes a number of patents on electrodes and multiple electrode systems. In the prior art multiple electrode systems, multiple electrodes or multielement electrodes are used to derive better estimates of the signal or to provide flexibility. Some multiple electrode systems are used to achieve rapid applications in emergency situations.
Prior art artifact detecting systems exemplified by Cudahy U.S. Pat. No. 3,905,364, Horth U.S. Pat. No. 3,552,386 and Vandenberg U.S. Pat. No. 3,050,841 detect artifact signals by sensing changes in the received signal out of range of a predetermined expected normal ECG from the same pair of electrodes used to derive the desired ECG signal; that is, between an electrode at a first ground or common location and an electrode at an ungrounded or uncommon location.
Holsinger U.S. Pat. No. 3,868,947 describes a multiple electrode system for artifact compensation and assumes a common mode artifact signal on the center electrodes of spaced coaxial electrode pairs in contact with the skin at spaced points with the outer annular electrode of each pair connected together and to a common ground of a differential amplifier having a pair of inputs respectively connected to respective ones of the center electrodes in a pair. This approach is of limited utility in solving the problem of detecting baseline shift because the causes of baseline shift at the location of one pair of electrodes is not likely to produce the same baseline shift, if any, at a spaced pair of electrodes.
Accordingly, it is an important object of this invention to provide improved methods and means for detecting baseline shift in an ECG signal.
It is a further object of the invention to achieve the preceding object with apparatus that is relatively easy and inexpensive to fabricate and install while providing a relatively reliable indication of baseline shift with relatively little attention to the apparatus.
It is a further object of the invention to achieve one or more of the preceding objects while providing a useful signal.
It is a further object of the invention to achieve one or more of the preceding objects useful in automatic ECG monitoring systems helpful in avoiding erroneous alarm conditions.